Donald Trump's ambiguous answer to a debate question on nuclear restraint raised doubts about his understanding of the issue. On the other hand, his words are by design or coincidence a mirror the nub of a policy argument the administration is wrestling with in the final months of Barack Obama's presidency.Asked at this week's debate whether he supports the decades-old U.S. policy of refusing to rule out being the first to use nuclear weapons, Trump at first said, "I would certainly not do first strike." That would seem to indicate he does not support the current policy of keeping it indefinite. But then he said, "I can't take anything off the table." And that would suggest just the opposite: that he would not rule out a nuclear first strike.It was difficult to tell whether Trump has considered this aspect of nuclear weapons policy. Or some others. During a Republican primary debate he was asked his view on modernizing the three main elements of the U.S. nuclear arsenal, known as the "triad," and he couldn't name all three: missiles launched from the air, underground silos and submarines.In his answer Monday he tossed in a mention of the B-52 bomber, which is part of the airborne leg of the nuclear triad. He correctly said the B-52 is extraordinarily old by weapons standards, and he said this shows the U.S. is "not keeping up" with other nuclear powers. The U.S. actually is planning to build a new-generation bomber and to replace all other elements of its nuclear arsenal.Questions about the circumstances in which the United States might use a nuclear weapon have resurfaced in recent months, as Trump opponents have openly expressed fear that he would use them unwisely, unleashing nuclear hell."A man you can bait with a tweet is not a man we can trust with nuclear weapons," Hillary Clinton said in her speech accepting the Democratic presidential nomination.Two Democratic lawmakers, Rep. Ted W. Lieu of California and Sen. Edward J. Markey of Massachusetts, said Tuesday they had introduced a bill that would prohibit the president from launching a nuclear first strike without a declaration of war by Congress.But Thomas Mahnken, president of the Center for Strategic and Budgetary Assessments, said he hopes Obama does not choose to adopt a "no first use" policy."Potentially it would be a president in the waning months of his administration seeking to tie the hands of his successor," he said.The administration in 2010 considered but rejected adopting a "no first use" rule on nuclear weapons, which in some respects would align with Obama's pledge to reduce the role of nuclear weapons in U.S. security policy. In recent months the administration has returned to the question but has announced no decision yet.The issue is complicated, but it boils down to this: What is to be gained by declaring in advance that the U.S. would not be the first to use a nuclear weapon? This was not an issue in August 1945 when the U.S. hit Japan with two atomic bombs, because no one else had the bomb.Those who favor a "no first use" policy say the prospect of first-use encourages Russia and possibly China to field a large portion of their nuclear forces on hair-trigger alert in order to avoid a disarming U.S. nuclear strike. That in turn increases the chances of nuclear war by accident or design.The counterargument is that "no first use" would undermine the confidence of U.S. allies in Europe and Asia that the U.S. would fulfill its treaty commitment to defend them.Clinton raised that point when she followed Trump's response at the debate. She did not say whether she favors a "no first use" policy but suggested she does not. After Trump's answer, she seemed to feel it necessary to reassure U.S. allies.The fact that it made her look more presidential perhaps played into her thinking as well."Words matter when you run for president. And they really matter when you are president," she said. "And I want to reassure our allies in Japan and South Korea and elsewhere that we have mutual defense treaties and we will honor them."Asked at a news conference Tuesday whether he favors a "no first use" policy, Defense Secretary Ash Carter made a similar point about alliance commitments, but not in the context of the political debate."It has been the policy of the United States to extend its nuclear umbrella to friends and allies, and thereby to contribute to the deterrence of conflict and the deterrence of war," he said. "Many of our friends and allies have benefited from that over time," and that should endure, he added.